doc : man page and admin-guide for newly introduced snapshot delete option.

Change-Id: Iab5e7f63d673a2040e8d83ab7280121ff468836e
BUG: 1112613
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8378
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>